Storage units are advertised by floor area: 5x10, 10x10, 10x20. But you don't fill a unit by floor area. You fill it by volume, stacking boxes and furniture upwards. Answering "what size storage unit do I need?" properly means working in cubic feet: how much space your belongings take up, and how much of a unit's volume you can really use.
1. Floor area vs volume
Most self-storage units have ceilings around 8 feet high, though some are taller. Check with the facility, because height changes everything. At 8 feet, the common sizes work out like this:
| Unit (ft) | Floor area | Volume at 8 ft high |
|---|---|---|
| 5 x 5 | 25 sq ft | 200 cu ft |
| 5 x 10 | 50 sq ft | 400 cu ft |
| 10 x 10 | 100 sq ft | 800 cu ft |
| 10 x 15 | 150 sq ft | 1,200 cu ft |
| 10 x 20 | 200 sq ft | 1,600 cu ft |
Some facilities quote unit sizes in feet and others in metres or square metres, especially in the UK, so convert before you compare prices.
2. Measure your belongings: a quick inventory
You don't need to measure every item precisely. Group things and estimate:
- Boxes: use the box maker's size, or measure length × width × height. A typical medium moving box is about 3 cu ft, and a large one about 4.5 cu ft.
- Furniture: measure each large piece as if it were a box around its widest points. A sofa 84 × 36 × 34 inches is 102,816 cubic inches, or about 59.5 cu ft.
- Appliances: use the outside dimensions from the manual or spec sheet.
Dividing cubic inches by 1,728 for a dozen items gets tedious, and it's easy to mix up inches and feet. A dedicated storage unit cubic feet calculator speeds this up: enter the dimensions of each item, or the unit you're considering, and it returns the volume in cubic feet.
3. Stacking height and aisle space: the real usable volume
You'll rarely use the whole volume of a unit. Heavy boxes shouldn't be stacked to the ceiling, sofas and tables leave awkward gaps, and if you need to reach things during storage, you need an aisle. In practice, plan on using roughly 60–70% of a unit's volume. If you're packing tightly for long-term storage with no access, you can go higher.
4. Worked example: a one-bedroom flat
Here's a rough inventory:
- 30 medium boxes: 90 cu ft
- 10 large boxes: 45 cu ft
- Sofa: 59.5 cu ft
- Double bed frame and mattress: about 60 cu ft
- Fridge-freezer (70 × 24 × 26 in): about 25 cu ft
- Washing machine (24 × 24 × 34 in): about 11 cu ft
- Dining table and four chairs: about 40 cu ft
- Bookcase (36 × 12 × 72 in): 18 cu ft
- Desk or TV unit: about 25 cu ft
That totals about 375 cu ft. Furniture never packs perfectly, so add about 25% for gaps, giving roughly 470 cu ft.
A 5x10 unit has 400 cu ft in total and only about 260 usable, so it's too small. A 10x10 has 800 cu ft and about 500–560 usable, which fits this load with a little room to spare.
5. Tips that save space (and money)
- Disassemble bed frames and table legs, and stand mattresses on their sides in covers.
- Use uniform boxes so they stack neatly and safely.
- Fill empty spaces, such as the inside of a fridge or wardrobe, with soft items (keep appliance doors slightly open to avoid mould).
- Put things you'll need at the front and label boxes on at least two sides.
For guidance on choosing a reputable facility, contracts and insurance in the UK, the Self Storage Association UK is a useful starting point.
Checklist
- Ask for the ceiling height and calculate each unit's volume.
- Estimate your items in cubic feet, and add about 25% for gaps.
- Plan to use about 60–70% of the unit's volume.
- Choose the smallest unit that fits, with room for an aisle if you need access.
